Distortion correction in transmission systems



Dec; 22, 1931'.

DISTORTION CORRECTION IN TRANSMISSION SYSTEMS` voriginal Filed oct. 5,1928 Ann "V q DEV/CE /NPDANCE DEV/CE /NVENTQR W F. MA .so/v

A7TOHNEY lli Patented Dec. 2v2, 1931 UNI-TED STATES PATENT OFFICE oWARREN P. iirAsON,` OF RAsT ORANGE, NEW', JERSEY, AssieNoR To BELLTELEPHONE LABORATORIES, INCORPORATED, or NEW YORK, N. Y., A CORPORATIONOP NEW Y YORK p iiIsT'oRTION CORRECTION liv-TRANSMISSION sYsTEMsOriginal application led October 3, 1928, Serial No. 310,165, and inCanada July 19, 1929. Divided and this application filed November 29,1929, Serial No; l110,350.

This is a division of my application Serial No. 310,165, filed October3, 1928 for distortion correction in transmission systems.

This invention relates to systems for transmitting electrical waveenergy, such as telephone transmission systems and the like, landparticularly to 'the correction' of distortion in such systems.

From an ideal quality standpoint, a transmission systemv should bedesigned so that the currents received thereover are exact copies inevery respect of the correspondingcurrentsentering the system atthe-transn'iitting end; in other words, the-transmission system shouldbe distortionless. When electrical Waves are transmitted over conductorsof relatively vgreat length,the distortion produced in the waves due tothe inherent properties of the conductors becomes of increas- -f ingimportance. In long, high quality systems, such as are now beingused fortelephone transmission, program broadcasting, television, etc. thedistortion produced by l lines is often of such magnitudev astoappreciably impair thequality of the receive(Y waves, and thus reducesthe commercial efiiciency of such systems.

Distortion in transmission' lines is, in general, of three differentkinds; first, attenuatio-n of the transmitted waves bv the line; second,an unequal degree of attenuation in waves of different frequenciestransmitted over the line;.and third, Va relative phase change in thetransmitted waves. The cause of such distortion is the improperproportioning of the resistance,gthe leakance, the capacity, and theinductance of the line.

The Pupin-Campbell and other loading methods haverbeen devisedwith theobject of reducing attenuation in the useful frequency range intransmission systems, Such loading systems havebeen supplemented inpractice `by amplifiers, such as the telephone repeater, to compensatefor reduction in the amplitude of the transmitted waves.y The secondtype of distortion inherent in transmission systems, an lunequal degreeof attenuation in the transmitted waves of different frequencies, may beovercome by the use of attenuation equalizingnetworks atdifferent pointsin the system, this expedient having been brought to a high state of-developinent. Phase corrective networks or phase conipensators tosupplement other types of distortion 'correctiomysuch as load-4 ing,have been used in connection with lines to Acorrect phase distortion inthe transmitted waves ofV adesired range of frequencies Other networkshave been devised and utilized in connectionwith transmission linestocombine the functions of phase distortion cor-4 rection and attenuationequalization 1n tra-ns` mitted waves of a desired range of frequencies.The latter networks have been designed,

in part at least, byV empirical methods, or else in' tlieirdesign allfactors contributing to distortion have not been taken into account.Their use, therefore, enables only an ap-V prOXimation of the idealdistortionless line to be obtained.

An object of the invention is to correct distortion invelectric wavetransmission.

Another and more specific object is to re-` duce the attenuation and tominimize varia-V tionsin the velocity of propagation of electric wavesover transmission systems.

In--accordance with-the invention, exact design Vformulaehave beendeveloped for the"V impedance relations which a structure must satisfy`when combined with a transmission line in a `given manner 'in order tocom# pletely compensate for all distortion includ?" ing amplitudey andphase distortion produced by the line on waves of all frequenciestransmitted thereover, and structures have been designed inaccordancewith these formulae to `accomplish the desired result; Afeature of the invention is a unitary structureor' network which whencombined with a transmissionV line will simultaneously compensate bothfor amplitude and phase' distortion in' waves of all frequenciestransmitted thereover, and at the same timer-estore the waves to theiroriginal amplitudes:

In accordance with the invention, the compensation for the amplitude andphasey disi tortion produced in waves of all frequencies transmittedover a given lengt-li of trans; mission line-is Obtained by inserting inthe lineV in tliepath of the' transmitted waves a structure havingcharacteristics equaling in value but effectively of opposite sign tothose of the given length of transmission line.

In a preferred embodiment this structure is inserted in series with thegiven line and comprises an open-circuited section of smooth line halfas long as the section to be compensated, and having a characteristicimpedance twice that of the section of line to be compensated when thestructure is not inserted therein, and in tandem therewith a devicewhich will effectively change the positive impedance of the insertedsection of line into a negative impedance of equal value.

ln another embodiment the compensation for phase and amplitudedistortion for waves of all frequencies transmitted over a section ofline, together with a desired degree of amplification, is obtained byinserting in shunt to said given section of line a structure comprisinga short-circuited smooth line of the same length as the given section tobe compensated, and having a characteristic imped ance half that of thesection of line to be compensated when the structure is not insertedtherein, and in tandem therewith a device which will effectively changethe positive impedance of the inserted section of line into a negativeimpedance of equal value.

Substantially distortionless overall transmission fer waves of allfrequencies over a transmission line of any length may be effectivelyobtained by inserting at intervals along the line in the path of theWaves, a structure such as described above, each structure beingdesigned in accordance with the principles of the invention tocompletely compensate for the amplitude and phase distortion in thetransmitted waves produced by adjacent portions of the transmissionline.

The objects and advantages of the invention will be best understood fromthe following detailed description when read in connection with theaccompanying drawings, in which Fig. l shows a diagram of a transmissionsystem embodying one form of the invention; Figs. 2 and 3 showdiagrammatically different forms of distortion correcting structureswhich may be used in the system of Fig. l; Fig. 4; shows a diagram of atransmission system embodying a modification of the invention; Figs. 5and 6 show different forms of distortioncorrecting structures which maybe used in the system of Fig. il; and F i0. shows a type of simulatingnetwork which may be used in place of the artificial lines in thestructures of Figs. 2, 3, 5 and 6 to approximate the results obtainablewhen said artificial lines are used.

In Fig. l is shown a transmission line l for transmitting alternatingcurrent waves of a plurality of frequencies generated by the source 2 toa load circuit represented by the resistance 3. The line 1 comprises aplurality of smooth line sections S1, S2, E53- of equal length and aplurality of series impedances, represented by the boxes ZA, insertedbetween the line sections each of these series impedances being designedin accordance with the invention to compensate for distortion in phaseand amplitude in the transmitted waves of all frequencies produced byone section of said line. The sections of line S1, S2, S3, etc. arerepresented in the ligure in well known manner by an equivalent Tresistance network having two series arms each of impedance I ZZ tanhand a shunt arm of impedance ZL smh where Z1 is the characteristicimpedance of the line l and I is the propagation constant for a sectionof the line of length Z.

The method of design of the series impedances ZA is as follows:

The propagation constant P for the complex line l including the smoothsections S1, Sg-and the series impedances ZA is given by the equation:

sinhl (l) Diiferentiating this equation with respect to w (where w=21rtimes the frequency of the transmitted waves) the following equation isobtained:

. The propagation constant P vof the line 1 1s composed of anattenuation constant A and a. phase constant B. Hence,

dP dA .dB d wjzgfygg (3) If @g dw equals Zero, then A and B areconstants independentof frequency. For this condition, the attenuationdistortion in the transmitted waves will be constant, and the velocityof the transmitted waves will be theoretically infinite since dd isequal to the delay.

If the left side of Equation (2) is set equal Y .ZA I 2z] t Zr0, thefollowing differential-'equation will be obtained: l -f Y where C isanarbitrary constant which is independent of the frequency. Substitutingthis value for ZA in Equation (1) itis found that coshP=0 ,(6)

Hence, if networks corresponding to Equation can be physically realized,Vand are inserted in line 1 inthe manner shown in Fig. 1, the line 1 maybe made such as to produce a Constant or Zero .attenuation and atheoretically'infinite or very high constant velocity in transmittedwaves of all frequencies.

If C=1, Equation 'reduces to Equation (7) may be physically realized.

in accordance'with the invention-by the structure of Fig. 2. Theelements ofthe structure of Fig. 2,'which in the system of Fig. 1 wouldbe inserted in the boxesv Z A in series with j. the terminals thereofcomprise a section of e, any suitable type, for example, a feed-backamplifier such as disclosed and claimed in the patent of R. C.`MathesNo.1,779,882, issued` October 21, 1930, which will effectively give betweentwo terminals thereof over a wide range of frequencies, the negative ofresistance, inductance, capacity, or complex combination thereof used asthe feed-back impedance@ If the structure ofV Fig. 1 of the li/lathespatent is used as the device 5 in the structure of Fig. 2 of thisapplicationthe terminals ZA of the latter structure would be connectedto terminals 16 and 1'? of the former structure, and the-two terminalsof net- .work 4 of the latter structure would be connected to theterminals of box 14 in the circuit of the former structure so thatnetwork a acts as the feed-back impedance Z insaid circuit. l

` The open-oircuited section of smooth line'll has been represented inFig.z2 bylits 'equiv-` alent -T network comprising two series armseachof impedance and a shunt arm of impedance 2Z, y sinh I" the elementsof these equations having the same significance as in the equationsgiven above. Y Y Y If a structure such as shown in Fig. 2 is utilizedfor the series impedances ZA in the Y system of Fig. 1, an overall zeroattenuation and Zero phase shift will be obtained thereinV fortransmitted :waves of all frequencies. i .IfC 1, Equationflreduces to:

ZZZO n sinh I (8) Equation (8) may be physically realized inaccordancewith the invention by thestructure shown in Fig. 3. Theelements of that structure, which in the system of Fig. 1 would beinserted inthe boxes ZA in series with the terminals thereof,com prise abranch 6 and a branch- 7 connected in series with the terminals of ZA.Branch (icomprises a section of open-circuited smooth line 8A of thesame length as, and with a characteristic impedance Vwhich is twice thatof the section of line 1 to be compensated, and in tandem therewith adevice 9, similar to the device 5 of Fig.- 2, which will change thepositive impedance of the open-circuited line 8 to a negative impedanceof equal value.

The branch comprises a section of opencircuited line 10, the same lengthas, and with `ZA v(th. F

a characteristic impedance which is 2C times that of the portion of line1 to be compensated,

and in series therewith a short-circuited section of smooth line 11 halfas long as, and with a characteristic. impedance which is V2C l timesthat of the section ofV line 1 to be comae pensated and in tandem withline 11 a device 12, similar to the device 9, Ywhich will eectivelychange the positive impedance ofV line 11 to a negative impedanceofequal value. The sections of open-circuited smooth line 1 8 and 10 andthe section of short-circuited line 11 are represented in Fig. S bytheir equivaw lent T networks having shunt and series arms. Each of thetwo vseries arms of the 4 equivalent T network of artificial line 8 hasan impedance of 'zoV soV

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.Each of the series arms of the T network representingartificial line 10has an impedance 2ozl ma Each of the series arms of the T networkrepresenting the artificial line 11 has an 11npedance of ZOZZ llanh gand the shunt arm an impedance of QO'Z, sinh F In Fig. l is shown atransmission system identical with that of Fig. 1, except that a shuntimpedance indicated by the box ZB, is utilized in place of each seriesimpedances ZA of the system of Fig. 1 between adjacent line sections tocompensate for amplitude and phase distortion in transmitted waves ofall frequencies for each section of the line 1. The method of designingthe shunt impedances ZB is as follows:

rEhe propagation constant for the complex line comprising the sectionsS1, S2, S3, etc. and the shunt impedances ZB is given by the equation:

cosh P cosh I -lsinh I lowing solution is obtained:

Z1 Slllh T ZBF O-coshI (11) Substituting this value for ZB in Equation(10), it is noted that cosh 13:0 (12) Hence, if the structure which willsatisfy Equation (11) is physically realizable and is inserted in line 1of Fig. -1 as the shunt impedances ZB in the manner shown, the complexline can be made to produce an overall uniform attenuation and atheoretically infinite or very high constant velocity in the transmittedwaves of all frequencies.

1f C equals 1, Equation (11) reduces to:

Zl/ sinh I ZB- 2 \1 cosh F) Equation (11) may be physically realized inaccordance with the invention by a structure such as shown in Fig. 5.The elements of this structure, which in the system of Fig. 5 would beinserted in the boxes ZB in series and the shunt arm of which has animpedance L 2 sinh F tanh g lf the structure of Fig. 5 is utilized forthe shunt impedances ZB in the system of Fig. 4, an overall zeroattenuation and a zero phase shift may be obtained therein fortransmitted waves of all frequencies.

If C 1 Equation (11) reduces to:

ZZ T ZL F t c+1 mnh B11-@Gah o f Zi t hr) 14) 2 (i+1/t2 an 2 Equation(141) may be physically realized in accordance with the invention by thestructure shown in Fig. 6. This structure, which in the system of Fig. 4would be inserted in the boxes ZB in series with the terminals thereof,comprise three different branch networks each connected directly acrossthe terminals of ZB. One of these networks comprises an open-circuitedsection of smooth line 15 of the same length as, and with acharacteristic impedance which is times that of the section of line 1 tobe compensated, and in tandem therewith a device 16, similar to thedevice 5 in Fig. 2, which will effectively change the positive impedanceof open-circuited line 15 to a` negative impedance of equal value.

rihe second of the networks in the structure of Fig. 6 comprises ashort-circuited length of smooth line 17 of the same length as, andhaving a characteristic impedance which is times that of the section ofline 1 to be compensated.

The third branch network in the structure of Fig. 6 comprises ashort-circuited length of line 18,half as long as, and with acharacteristic impedance which is one-half that of the section of lineto be compensated, and Vin tandem therewith a device `19, similar to thedevice'l, WhichWill effectively change the positive impedance of theshort-circuited line 18 to a negative impedance of equal value.

The open-circuited line 15 is represented in Fig. 6 by its equivalent Tnetwork, each of the series arms of which has an impedance of Theshort-circuited line 18 is represented in Figli by its equivalent `Tnetwork, each of the two series arms of which has an impedance of @cuiZ, Y I tainh and the shunt arm of which has an impedance If thestructure of Fig. 6 is utilized for the shunt elements ZB in the systemof Figli, an overall uniform attenuation and a zero phase shiftmay beobtained therein for transmitted waves of all frequencies.

A simulating network made up of inductances, resistances and capacities,suoli asgillustrated in Fig. 7, may be utilized in place of each of theAartificial lines of the Vstructures of Figs. 2, 3, 5 and 6 toapproximate the results obtainable when the artificial lines are used.This simulating network is a T structure, each of the series arms VofWhich comprises a resistance of value Y 2 and an ind'uctance of value 2in series, and the shunt arm, a capacity of Y valueC and a resistance ofvalue inductance, the capacityand 'G the leak-j ance Vof the section 'ifline to be compensated bythe compensating. structure. f

- It should be noted that each impedance ele` ment inthe structures t ieinvention Which have` been illustrated and .described contributes-bothto attenuation distortion correction Vand phase distortion connectionthisl being an important distinguishing` feature from the structures ofthe prior art inWhich Vattenuation correction or phase distortioncorrection is the sole function Vof separate elements therein.. f

Althoughihe invention has been directed- Y principally to the obtainingof complete com-Y pensation Vfor attenuation and phase distor tion, .it-is apparent that it may be utilized for yobtaining any desired amountof attenuation:

and phase distortioncorrection. i o

Although the Vinvention has been described in connectionwith aparticular transmission ysystem comprising a plurality-of uniform`smooth sections of line and a plurality` of distortion compensatingVstructures spaced `at equal intervals along4 a transmissionline, theinvention is notso limited, but is applicable to a system comprising asmooth or a loaded. linein combination with a single distortioncompensating structure, or a plurality of dis'- tortion compensatingstructures spaced at unequal Vintervals alongV vthe line, or any, othercombination Within the scopeof the appended claims. Y

`What is claimed is: o i

1. ln combination With 'a transmission line forA transmittinglelectricalWaves of difer- Y ent frequencies, means for effectively compensatingforall distortion both in phase and amplitude produced on transmittedWaves of. allfrequencies by a'given portionof said line, said -meanscomprising an electrical structure in shuntl With said given portion ofsaid line, and having characteristics equivalent in value to those ofsaid given portion of line but of opposite sign. Y

2. In combination with a transmission line for transmittingelectricalrwaves of diiier ent frequencies, means for eiiecti'velycompensating for all distortion produced by said line in the transmittedWaves of all frequencies including distortion in phase and amplitude,said means comprising an electrical structure inserted in shunt withsaid line and traversed by saidwaves, said structure having apropagation constant and a character istie impedance substantially equalto that of vsa'idline but impedance characteristics VWhich areequivalent in value to those of said lineV but of opposite sign. Y Y i3. YIn combination, a transmission line for transmitting electricalWaves of diii'erent frequencies, and Vmeans for compensating for alldistortion in-.amplitude andphase produced in said Waves ofdiiierentirequencies by a given section of said line, said means comsinsoif

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prising an electrical structure inserted in shunt with said given linesection and traversed by said waves, said structure comprising a networkhaving impedance characteristics equivalent to those of ashort-circuited smooth line the same length as said given line sectionand with acharacteristic impedance half that of said given line section,and in tandem therewith a device which will effectively change thepositive impedance of said network to a negative impedance of equalvalue.

4. A combination in accordance with that of claim l, in which saidelectrical structure in shunt with said given portion of said line`comprises a short-circuited smooth line the same length as said givenportion of said line and having a characteristic impedance half that ofsaid portion of line before said structure was inserted therein, and intandem therewith a device which will eectively change the positiveimpedance of said shortcircuited line to a negative impedance of equalvalue.

5. In combination, a transmission line for transmitting electrical wavesvarying in frequency, and means for compensating for phase and amplitudedistortion in waves of all frequencies transmitted over a given portionof said line, said means comprising a structure inserted in shunt withsaid portion of line having an impedance of Zz I tallll where Z1 is thecharacteristic impedance and Iis the propagation constant of saidportion of line before the' insertion of said structure therein.

In Witness whereof, I hereunto subscribe my name this 27 day ofNovember, 1929.

WARREN P. MASON.

